Power-shifting automatic transmissions of both the planetary type and the countershaft type use hydraulically actuated torque transfer devices to effect the selection of sequential drive ranges by selectively engageable friction members. Planetary type transmissions use friction torque transfer devices of both the clutch and brake variety. Countershaft type transmissions use friction torque transfer devices of only the clutch variety. The control mechanism which determines the shift sequence and timing for these transmissions can be either hydraulic control valving or the more recently introduced electro-hydraulic control valving. With electro-hydraulic controls, a pre-programmed digital computer is generally provided to determine both the shift schedules and pressure levels of the hydraulic actuating fluid within the transmission. The computer employs a look-up table which has the necessary data to determine the shift points in response to input signals from vehicle parameter detectors, such as the vehicle and engine speed sensors, engine torque level sensors, throttle position sensors and the like.
The computer analyzes the input signals and refers to the look-up table to determine the appropriate ratio interchange. The computer can also provide the necessary control signals to establish the desired output pressure of the solenoid valves. Generally, the solenoid valves are either of the on-off type or the pulse width modulated (PWM) type. With either type, the output signal is delivered to either a valve, which will control the ratio interchange, or to the friction devices directly.
Currently employed control devices typically utilize a governor and throttle signal to control the ratio interchange. In some instances, this signal is combined by the electronics to provide a single electrical output signal which will determine the output pressure of the solenoid control valving. Should the solenoid valve have a malfunction, the transmission control includes a limp-home feature which causes the transmission to select a fixed gear ratio until proper repairs are undertaken. This feature prevents the driver from being stranded due to an electrical or mechanical malfunction of the solenoids.
The purpose of the governor valve is to direct the higher of the two solenoid pressures to the governor pressure passage and to the boost side of a plug valve incorporated in the trim boost valve. The governor valve also directs the lower of the two solenoid pressures to the other side of the plug valve in the trim boost valve. Trim boost pressure is maintained at a level determined by a spring that is set by the differential pressure between solenoids which act on the plug through a pin and stop structure. During upshifts, one solenoid is operated at a lower level than the other solenoid so that a differential pressure exists on the trim boost plug resulting in the desired trim boost pressure. Between upshifts, the pressure of the solenoid operated at the lower level rises to the same level as the other solenoid. Interlock valves provide the control that will "lock-out" a solenoid if a malfunction occurs which provides a pressure continuously greater than zero. The "lock-out" is introduced when the transmission is shifted to reverse, thereby ensuring that the vehicle speed is essentially zero to prevent any unscheduled downshifts. The control will permit the operator to resume normal operation although the upshifts will be harsh because of the high trim boost. This will remind the operator that some service is needed.
The control devices currently known, such as depicted in U.S. patent application, Ser. No. 08/073,238, filed on Jun. 7, 1993, in the name of Long and assigned to the assignee of the present invention, have a separate interlock valve for each of the solenoid valves. The use of separate interlock valves provides highly satisfactory operation. However, such a system not only requires more space to house the control but also incurs an increased cost to provide the larger housing and the additional interlock valve.